The present invention relates to a method and apparatus for molding a plastic pipe, which is particularly suitable for forming a plastic pipe of complicated configuration.
Since a pipe used as an air duct for a ventilation device or cooling device must generally be disposed at a position subject to different constraints, a rubber pipe rich in deformability, or flexibility, has been conventionally used. If the material of a pipe has poor flexibility, various kinds of pipe joints must be prepared to complete a piping system.
Although a rubber pipe can be used with excellent performance and easy handling, its exorbitant production cost and difficulty in forming a pipe in complicated configuration inhibit wide spread application. Under the circumstances, attention has recently shifted to the use of a less expensive thermal plastic resin such as polyethylene, polypropylene and the like as an alternative material. When use is made of a thermal plastic resin, however, there are still disadvantages mainly caused by the difficulty in forming a plastic pipe of complicated configuration in accordance with the conventional technology. For example, a number of plastic pipes of different sizes and shapes together with various kinds of joints such as elbows must be prepared. It is quite time consuming to assemble these pipes and joints to set up a desired piping system. Furthermore, it is often the case to leave small interstices at a joint, which allow the passage of fluid.
Explaining the conventional technique to form an elbow pipe with reference to FIGS. 1 to 3, a parison 13 in the form of a hollow tube is continuously extruded out of a nozzle 14 vertically downward, i.e. in the direction of gravity, into the position where vertical mold halves 12a and 12b, each having an engraved groove 11a or 11b on its surface, are oppositely placed. Each of the grooves 11a or 11b has a semi-circular cross section so that when the mold halves 12a and 12b are brought into contact, a mold cavity in the form of an elbow pipe is defined therebetween. Then the mold halves 12a and 12b are brought into contact under pressure, followed by blowing air into the parison. Upon cooling, an elbow pipe is formed in compliance with the shape of the grooves engraved in the mold halves.
In accordance with this conventional technique, however, since the parison 13 is hanging down from the nozzle 14 under gravity between the vertically arranged mold halves, there is a limit in length and it is virtually impossible to control its thickness. If the parison 13 gets too long, it will change its shape or simply tear off. Moreover, if a pipe to be formed is complicated in structure, it will necessarily produce a flash by overflowing the mold cavity because excessive material must be supplied. This requires a secondary processing of flash removal which, in turn, increases production cost. For example, even in the case of molding an elbow pipe as explained above, somewhat excessive material must be supplied to fill the cavity completely. When the mold halves 12a and 12b are pressed against each other, the excessive material overflows into the gap between the mold halves 12a and 12b to produce a flash 16. The production of such a flash is just waste of material as well as requiring a secondary processing of flash removal, which can be quite laborious if the pipe is complicated in configuration. Therefore, in accordance with such a conventional plastic pipe molding technique, it is impossible to form a pipe of complicated configuration, e.g. a pipe provided with bellows in the middle to give flexibility in bending, a pipe with a sudden area change, or a pipe with its shape changing three dimensionally.